For the past several years, considerable effort has been expended on the development of a miniature fiber optic pH probe for physiological use. With the first generation of probes and support equipment the feasibility of optically measuring pH via a pH sensitive dye was demonstrated and many subtleties associated with the probe construction and use made apparent. With this knowledge and experience and improved probe and multichannel support system has been designed and constructed and is presently being used to measure pH in the wall of beating canine hearts.